While activation of the hypothalamic-pituitary-adrenal (HPA) axis is an adaptive response to stress, excessive HPA axis reactivity may be an important marker of childhood vulnerability to psychopathology. Parenting, including parent affect during parent-child interactions, may play an important role in shaping the developing HPA system; however, the association of parent affect may be moderated by child factors, especially children's emerging self-regulatory skills. We therefore tested the relationship between parent affectivity and 160 preschoolers' cortisol reactivity during a laboratory visit, examining children's effortful control (EC) as a moderator. Greater parent negative affectivity was related to greater initial and increasing cortisol over time, but only when children were low in EC. Higher parent positive affectivity was related to a higher baseline cortisol for children with low EC and lower baseline cortisol for children with high EC. Results indicate that children's EC moderates the extent to which parent affect shapes stress reactive systems in early childhood. (Contains 3 figures, 2 tables, and 1 footnote.)